Finding Eb from C step by step:

The Maj 6th

The major sixth of a C Diminished 7th chord is
A. The major sixth is down three half-steps from the
Root (a minor third down).

In diminished chords, the major sixth is also known as the diminished seventh. For the purposes of this tutorial,
we'll be learning it as the major sixth, since it's easier to consider it separately than the other two sevenths,
the major seventh (M7) and minor seventh (m7)